optimize-fslot-value-switch

As with all compiler switch variables, the value of this variable
can be t, nil, or a
function object that accepts five arguments and returns t or nil. The arguments
passed to the function will be the values of the safety, space, speed,
debug, and compilation-speed optimization qualities, in that order. nil is equivalent to a function that always returns
nil and t to a
function that always returns t. When the
value is a function and we say t (or true) or
nil (or false) in the text below, we mean
that the function returns, respectively, t or
nil.